A flow cell is an apparatus for characterization of particles suspended in a sample solution. Particles sizes are generally in the range of ˜0.5-40 μm. Particles are analyzed one-by-one with a typical count rate in the range of a few to thousand particles per second. Depending on its configuration, a flow cell could allow estimating different information about the particles such as presence, concentration, dimension, shape, vitality (in the case of cells), types of biological cells, structural and/or functional information, etc. Using a flow cell for sorting particles of different types in a heterogeneous solution is also possible. An example of a flow cell is described in International Application no PCT/CA2013/000565 to Alain Chandonnet, Michel Fortin and Dany Nolet, filed on Jun. 12, 2013, the disclosure of which is incorporated by reference herein.
Flow cytometers, which incorporate different configurations of flow cells, have been developed over the last 40 years. In general, a light source (i.e. a laser) emitting a light beam is focused on a fluid stream in the flow cell. The fluid flows at a predetermined rate in a capillary tube of the flow cell. Particles in the fluid stream cross the light during a brief interval of time, hence forming a short burst of temporal scattered and fluorescence light. A collection optics assembly, localized near or around the region where light and fluid intersect collects light emitted and/or scattered by the particles. The collected light is spectrally separated by a detection subassembly system, including for example various optical filters, and then received by detectors. Optical signal parameters of the collected light are measured by the detectors, and are processed by a computational system and/or electronic components.
In one particular configuration, the flow cell includes an excitation fiber for transporting an excitation light generated by the light source. The excitation fiber comprises a passageway, for allowing the fluid to flow through the excitation fiber and thus allowing the particles in the flow to interact with the excitation light. The flow cell also includes at least one collection fiber for collecting light scattered or emitted by the particles flowing through the passageway and excited by the excitation light. In this particular configuration, the use of a capillary tube for fluid injection into the passageway of the excitation fiber is necessary to avoid compromising the characteristics of the collection fiber(s) and the overall performances of the flow cell.
Furthermore, immersion oil is generally used for index matching between the excitation fiber, collection fiber(s) and the capillary tube, to minimize stray light due to numerous optical interfaces and block generation of auto-fluorescence and spontaneous Raman scattering which can limit sensitivity. The immersion oil can be removed easily if in contact with water (for instance during rinsing of the capillary tube), rendering the flow cell unusable.
Although the capillary tube allows maintaining the characteristics of the collection fiber(s) and the overall performances of the flow cell, its use has several drawbacks. First, because the capillary tube is relatively small in diameter and have a certain length, it can be clogged by the particles in the sample, thus becoming inoperative. Some mechanisms permit rinsing the capillary tube, but again, due to its relative size and length, pressure of the rinsing liquid must be maintained within safe limits. Furthermore, the use of the capillary tube together with the excitation fiber and the collection fiber(s) require precise relative adjustment to ensure proper functioning of the flow cell. As the capillary tube, excitation fiber and collection fiber(s) are small components, replacement of a capillary tube in the flow cell is not a simple task which can be performed quickly, but rather requires concentration and precision. Immersion oil for epifluorescence microscope must be used between the capillary tube and the oil without excess (˜nl). The capillary tube must be glued at both extremities without blocking the entrances. Care must be taken during the assembling process because of the fragility of the capillary tube. Also, even immersion oil for epifluorescence microscopy can generate autofluorescence and spontaneous raman scattering.
There is therefore a need for an improved flow cell for characterizing particles in a solution, to mitigate or eliminate these drawbacks.